Microstructure and mechanical characteristics of an in-situ synthesis of AA7075/TiB2 metal matrix composite

In the present study, AA7075/TiB2 aluminium metal matrix composite (AMCs) was prepared by stir casting method using in-situ reactions of inorganic salts KBF4 and K2TiF6. In this process AA7075 alloy is reinforced with different weighted percentages of (5 %wt, 10 %wt, and 15 %wt) Titanium Diboride (TiB2) particles. X-ray Diffraction (XRD) investigation reveals the presence of TiB2 particles without any formation of the intermediate phase. An optical microscope was used to examine the microstructure, which revealed that the TiB2 particles are equally distributed and that grain size reduces as the weighted percentage of reinforcement particles increases. When the weighted percentage of TiB2 reinforcement particles increased, the microhardness and ultimate tensile strength of the AA7075/TiB2 AMCs increased. Furthermore, the ductile mode of failure of the tensile specimen has been observed by fractography analysis.


Introduction
Metal matrix composites (MMCs) are composites made up of metallic matrix (copper, magnesium, cobalt, iron, and aluminium) and a scattered phase. The primary benefits of aluminium-based MMCs materials are extraordinary strength, controlled thermal coefficient, wear resistance, improved stiffness and improved abrasion, and enhanced damping capabilities. [1][2][3] Aluminium-based metal matrix composites are the composites where Al is used as matrix, and the other components are inserted into the matrix, which is known as reinforcement, for example, (TiB2, SiC, Al2O3, C, B4C,). Al-based MMCs are broadly utilized in automobiles and Aerospace applications. It is because of its properties like high strength, improved thermal properties, and improved durability [8]. Particulates reinforced aluminium matrix composites (PRAMCs) feature a unique set of properties, including high thickness, high hardness, high specific modulus, and a low magnification coefficient at low temperatures [10]. Because of its high hardness, great thermodynamic stability, high corrosion resistance, high modulus, high melting point, and low density, the TiB2 class is considered to be an excellent reinforcement in aluminium among the various possible reinforcement particles such as Al2O3, Si3N4, TiC B4C, SiC, and TiB2. Aluminiumbased MMCs are considered to synthesize monolithic material, including titanium alloys, aluminium alloys, polymer composites and ferrous alloys in numerous applications. In this investigation, stir casting was chosen over other processes like powder metallurgy, foil diffusion bonding, stir casting, and physical vapour deposition.
In the casting process, a liquid substance is poured into a mould, which contains a hollow chamber of the desired shape, and then allowed to harden. Stir casting is a sort of casting method in which a mechanical stirrer is used to create a vortex in the matrix material to combine reinforcement. The main advantages of stir casting are its simplicity, flexibility, and possibility of producing large volumes. This process is the most economical among all the available approaches. It also allows a substantial component to be fabricated.
In-situ composites are multiphase materials in which the reinforcing phase is produced within the matrix while the composite is being made. Ex-situ composites are those in which the reinforcement is synthesized outside of the matrix and then incorporated in the matrix using a secondary process like penetration or powder metallurgy. [9] Another method was built in this study to manufacture TiB2 particulate aluminium in-situ composites with enhanced molecular dispersion, which included mechanical stirring on the salt/aluminium interface. Typical work on changing aluminium-based compounds (ex-situ composites) entails adding reinforcement to matrix alloys from the outside, such as SiC, Al2O3, and TiC. This mechanism can result in thermodynamic instability of the reinforcement and poor interfacial attachment. In-situ composites have a more homogenous dispersion of the scattered phase particles because the reinforcement is combined inside the matrix. The bonding between in-situ-shaped dispersed particles and matrix is better than in ex-situ MMCs. Types of equipment for preparing in-situ composites are less expensive than the other. Many researchers have explored the impact of the weighted percentage of TiB2 particles on the microstructure and mechanical behavior of composite material and saw that as the weighted percentage of reinforcement particles changes, the mechanical and microstructural behavior also changes. Soltani et al. [5] studied Al-SiC composites (3% wt) and found that a shorter stirring period is needed for ceramic amalgamation to achieve metal/ceramic bonding. Srivatsan et.al [6] investigated the influence of particulate silicon carbide on the mechanical properties of Al 6061 MMC. Sethi et al [7] studied the synthesis of AA7075/TiB2 aluminium composite and found that the ultimate tensile strength increases, hardness increases, and the size of grains decreases with an increase in the weighted percentage of the reinforcement particles.
In the current investigation, AA7075 alloys were chosen for matrix material and TiB2 as reinforcement with different weighted% (5%wt, 10%wt, and 15%wt). Synthesis is done through an in-situ reaction of KBF4 and K2TiF6 salt by the stir casting method. The impact of reinforcing particle weighted percent on microstructure and mechanical qualities is investigated.

Experimental Procedure
In the present study, AA7075/TiB2 composite was synthesized by the bottom pour stir casting method. Casting is done in vacuum condition in the presence of Argon gas. An advanced ultrasonic vibrator is used as a stirring mechanism for the uniform distribution of reinforcement particles. Inside a graphite crucible, AA7075 rods were placed and then heated using an electrical furnace. To remove the moisture content a combination of K2TiF6 and KBF4 salt of required weighted% was preheated at 250 ƕ& IRU mins. On molten aluminium alloy, preheated salts were added at ƕ& DQG VWLUUHG intermittently for 40 mins. After complete expulsion RI VODJ DW WHPSHUDWXUH ƕ& WKH composite melt was bottom-poured in the preheated die.
Some gases and intermetallic compounds Al3Ti and AlB2 were formed when KBF4 and K2TiF6 salts were added with molten aluminium. Again, Al3Ti and AlB2 react with each other to form TiB2 particles. For the complete conversion of TiB2 and Al3Ti, an excess amount of KBF4 was added. In Fig.1. An experimental setup of stir casting is shown. Using the standard metallographic technique, the specimens were polished. Etching of specimens was done by using Keller's reagent. For microstructure analysis of etched specimens, a scanning electron microscope and Leica light optical microscope was used. The tensile samples were produced according to the ASTM-E8 standard for tensile strength testing. In the INSTRON 8801 UTM, the specimens were evaluated at a strain rate of 0.5mm/min. The composite's microhardness was tested using Vicker microhardness estimation with a 100g load and a 30s dwell duration. Scanning electron microscopy was used to investigate the fracture surface of the failed tensile sample (SEM). In Fig.  2. The weighted percentages of the as-cast AA7075/TiB2 composite are depicte

X-Ray Diffraction (XRD) analysis of composite material
The in-situ formation of the TiB2 phase is formed during the synthesis of KBF4 and K2TiF6 salts and molten aluminium. In Fig 3 X-Ray diffraction (XRD) pattern of the AA7075/ TiB2 reinforcement composite specimen is shown. The complete reaction has been observed as no intermediate phase is formed only Al and TiB2 are present. This indicates that the TiB2 particles insitu are thermodynamically stable. Undesirable compounds will be produced by reaction with matrix alloy if particles are not in a thermodynamically stable state. The absence of other compounds in significant quantity indicates that the interface between AA7075 and TiB2 is free from contamination. From the pattern, it is also observed that as the weighted percentage of reinforcement particles increases, the intensity of peaks of TiB2 also increases. This will eventually accumulate at the particles-matrix alloy interface, lowering the load-bearing capacity. It is also seen that the AA7075 diffraction peaks were higher because of the presence of a large quantity of matrix material in the composites.

Microstructure of AA7075/TiB2 AMCs
Microstructures of varied weighted percentage (5% wt, 10% wt, and 15% wt) reinforcement AA7075/TiB2 are illustrated in Fig.4. Both intergranular and intragranular zones have an equal distribution of TiB2 reinforcement particles. The microstructures of as-cast AA7075 are dendritic structures, these dendritic structures are formed due to solidification [8]. The dendritic structure of cast AA7075 has been adjusted to grainy structures, this is because of the grain refining activity of TiB2 particles [12,13]. From the figure below, it can be seen that the amount of agglomeration in 10% wt and 15% wt reinforcement is insignificant. Because the nucleation sites grow, grain formation of the aluminium matrix is restricted, the size of the composite grains reduces as the weighted percent of reinforcement increases [10,11]. The presence of TiB2 particles, which increases the nucleation rate, is also seen in the microstructure, indicating that the composite is better attributed to their existence. Synthesizing TiB2 particles within the melt minimises the chance of oxidation, resulting in a cleaner and stronger interface.

Microhardness analysis of AA7075/TiB2 aluminium matrix composites
The microhardness of various weighted percent (5%, 10%, and 15%) reinforcement as-cast AA7075/TiB2 AMC was displayed in Fig. 5. It was seen that as the weighted% of TiB2 reinforcement particle increases, the microhardness of the composite also increases. This is due to the fact that the rigidity of ceramic reinforcement increases as the weighted percent of TiB2 reinforcement particles increases. The improved hardness of the composite is due to the good amalgamation between matrix and reinforcement, as well as the UHLQIRUFHPHQW ¶V fine size and uniform distribution. An increase in density of dislocation occurs as a result of the decrease in grain size which improves the resistance to plastic deformation [11]. The improvement in mechanical property is due to fine grain, according to the Hall-Petch relationship [14]. 5%wt reinforcement as-cast AA7075/TiB2 had a minimum microhardness of 100 HV0.1, 10 %wt reinforcement as-cast AA7075/TiB2 AMCs had a microhardness of 130 HV0.1 and 15 %wt reinforcement as-cast AA7075/TiB2 AMCs had a maximum microhardness of 150 HV0.1.

Tensile strength analysis of AA7075/TiB2 aluminium matrix composite
A tensile test was used to determine the mechanical characteristics of an AA7075/TiB2 aluminium matrix composite material with varied weighted percents of reinforcement particles. The ultimate tensile strength (UTS) of 5 % wt, 10% wt, and 15% wt reinforcement is illustrated in Fig. 6 (a). The ultimate tensile strength of the composite was found to grow as the percent weightage of the reinforcement increased. It is because of the increase in the quantity of high-strength TiB2 particles. This is also due to the presence of reinforcement particles which have a different coefficient of thermal expansion property than the matrix composite. During the deformation of particulate reinforced composites, the matrix material bears the majority of the load; however, the reinforcing particles within the matrix bear the same stress and prevent matrix distortion. When dislocations bind to particle-matrix interfaces, the mobility of the dislocations is halted, resulting in stress concentration at the particle level. In Fig.6 (b) the percentage elongation of 5 wt%, 10 wt% and 15 wt% reinforcement is shown. The percentage elongation diminishes with the increment in wt% of reinforcement. This is because of a decrease in ductile matrix content when wt % of TiB2 particles increases in the matrix material. A decline in pecentage elongation of AA7075/TiB2 aluminium matrix composite likewise happens because of an increase in hard ceramic content. Ranjan et al. [8] also reported similar results. Minimum elongation of 3% was obtained in 15 wt percent reinforcement as-cast AA7075/TiB2 AMC, 3.2 percent in 10% wt of reinforcement, and 4.9 percent in 5 %wt of reinforcement of as-cast AA7075/TiB2 AMC.

Fractography analysis of AA7075/TiB2 AMCs
The wrecked tensile specimens were examined in an electron microscope to investigate the reason for failure and fracture morphology. The fracture surface of the composite is shown in Fig.7 at various weighted percents of reinforcement (5 % wt, 10% wt, and 15% wt). Brittle fractures of AA7075/TiB2 composite have been observed macroscopically and ductile fracture has been observed microscopically. The void size of the AA7075/TiB2 composite was reduced during in-situ synthesis. This is due to TiB2 reinforcement's grain refining activity. The ductile behavior of the fracture surface in A7075/TiB2 material reduces as the grain size decreases. It was observed that in composite with 5wt% reinforcement, the fracture surface has voids with dimple indicating a ductile failure. In composite with 10wt% of reinforcement, large shape voids with fine dimple have been observed which indicates the failure in ductile mode. In composite with 15wt% of reinforcement, dimple shaped void with layered morphology has been observed which indicates failure in ductile mode.

Conclusions
AA7075/TiB2 AMCs were made in this study using insitu reactions of KBF4 and K2TiF6 salts. The impact of TiB2 particles on the microstructural and mechanical properties of AA7075/TiB2 AMCs has been thoroughly investigated. The following are some of the inferences that can be drawn. x The AA7075/TiB2 composite was effectively produced using TiB2 reinforcing particles of 5%, 10%, and 15% by weight. There is no intermediary phase formed and only Al and TiB2 are present, indicating that the reaction is complete. x It can also be seen in the XRD pattern that when the weighted percent of reinforcing particles increases, the intensity of TiB2 peaks increases. x The reinforcement particles are evenly dispersed throughout the matrix, and the grain size decreases as the percentage weight of TiB2 reinforcement particles increases. x The ultimate tensile strength of the composite has improved as the percent weight of reinforcement has increased. x Microhardness has increased as the percent weight of reinforcement in the composite has increased. x Fractography examination of tensile specimens revealed a ductile mode of failure.